1. Field of the Invention
This invention relates to a solid phase extraction sheet material that includes a porous matrix prepared by a wet-laid method, active particulate entrapped therein, and a polymeric hydrocarbon binder.
2. Background Information
Fibrous pulps can be converted to sheet materials by classic paper making methods generally known in the art as xe2x80x9cwet-laidxe2x80x9d or xe2x80x9chandsheetxe2x80x9d methods. In addition to the preparation of paper from a cellulose pulp, the preparation of sheet materials useful as substrates for physical filtration processes and paper chromatography is also known.
Addition of particles to such fibrous pulps is also known. In U.S. Pat. Nos. 5,300,192 and 5,352,480, wet-laid sheet materials that contain organic or inorganic particles capable of binding to an added binder compound (through covalent or hydrogen bonding) are described.
Synthetic papers prepared from poly(m- or p-phenylene iso- or terephthalamide) fibers, widely known as aramid fibers, are known. Such aramid papers are commonly processes into dense, non-porous, stiff substrates such as substrates for circuit boards. See, e.g., U.S. Pat. No. 4,729,921.
Aramid papers that include fillers have been described in, for example, U.S. Pat. No. 4,548,678. Fillers are often used to alter the physical properties or performance of the aramid paper. Commonly, filler-containing papers also are dense and non-porous.
A high porosity aramid paper that includes pulp from both fresh and previously dried (i.e., used or recycled) fibers but that does not contain particles or binder is described in U.S. Pat. No. 5,026,456. Other porous aramid papers that do not contain particles or binder, but which are useful in physical filtration processes, are described in U.S. Pat. Nos. 4,524,103 and 5,529,844.
A wet-laid aramid sheet material that contains reactive or sorptive particles is described in PCT Patent Document WO 95/17247. This sheet material is prepared in the absence of any binder and, accordingly, exhibits limited physical strength and tends to shrink when wet.
Crowder, III et al. (U.S. Pat. No. 4,384,957) describe a chromatography column in which a series of fiber-particle matrices are configured in a stacked arrangement. Suitable fibers are said to include polyacrylonitrile, rayon, cellulose, etc., and useful particles are said to be those that xe2x80x9cexhibit chromatographic functionalityxe2x80x9d (e.g. silica, alumina, activated carbon, ion exchange resins, etc.). In passing (col. 10, lines 58-61), the use ofxe2x80x9cchemical bindersxe2x80x9d is provided for, although none of the examples teach the use of a binder and no type of useful binder is mentioned in the document.
Not yet described in the art is a wet-laid type sheet material that contains reactive or sorptive particles and that possesses high wet strength, good tensile strength, and pleatability.
Briefly, the present invention provides a solid phase extraction sheet (i.e., a sheet that can remove one or more chemical species that are dissolved or entrained in a fluid) that includes a porous polymeric pulp comprising fibers, at least 75% of which are at least 4 mm long; from about 3 to about 7 weight percent of a polymeric hydrocarbon binder; and particles entrapped in the fiber pulp. A plurality of the particles exhibit reactive and/or sorptive properties toward the chemical species to be removed, and the weight ratio of particles to binder is at least 13:1. The sheet has a thickness of up to about 5 mm, a basis weight of from about 600 to about 2000 g/m2, and an apparent density of at least about 0.35 g/cm3.
Solid phase extraction (SPE) sheets can be used in place of particle-packed columns to remove solutes from fluids. Accordingly, the inclusion of large amounts of active (i.e., sorptive or chemically reactive) particles in such sheets is desirable. In the SPE sheet of the present invention, particles are present in such an amount that the weight ratio of particles to binder is at least 13:1, preferably at least 14:1, and more preferably at least 15:1. Preferably, a plurality of the particles are active particles. A preferred amount of binder is about 5% (by weight). When this amount of binder is used, the amount of particles is about 65% (by wt.), preferably about 70% (by wt.), and more preferably about 75% (by wt.).
The average uncalendered thickness of the sheet can be up to about 5 mm so as to keep extraction efficiency as high as possible. Nevertheless, if very small particles are used, the pressure drop across the SPE sheet can become unreasonably high, so the sheet preferably has an average uncalendered sheet thickness is at least 125 times larger, more preferably at least 175 times larger, most preferably 200 times larger, than the effective average particle diameter. Also, to ensure that the sheet can be pleated by commercially available pleaters such that the edges of the folded sheet do not substantially crack (so as to permit channeling or leaking), the basis weight of the sheet preferably is about 600 to about 2000 g/m2 and the apparent density preferably is at least about 0.35 g/cm3, more preferably at least about 0.4 g/cm3.
The SPE sheet of the present invention can be used in severe conditions where currently available SPE sheets, e.g., fibrillated polytetrafluoroethylene (PTFE) sheets, tend to degrade. An example of such severe conditions is in the highly caustic (i.e., pH of about 14), radioactive isotope-laden waste from some nuclear facilities. The radiation emitted by the radioactive isotopes of such waste (e.g., Cs+ and Sr+2) degrades PTFE SPE sheets.
Unless a contrary intent is evident, the following definitions apply herein:
(a) xe2x80x9cactivexe2x80x9d means possessing ion exchange, reactive, or sorptive properties;
(b) xe2x80x9capparent densityxe2x80x9d means the weight of a given SPE sheet divided by its average volume (which can be calculated by measuring the average thickness of a sample of a sheet of known length and width); and
(c) xe2x80x9chydrocarbonxe2x80x9d (when used in conjunction with binders) means an organic material that may contain heteroatoms (e.g., O, S, N, F, etc.) but derives at least 50% of its weight from hydrogen and carbon.